The mobile shells all use the same isolate snapshot. This is also the snapshot used by the service isolate. This works towards a world where the isolate snapshot is no longer a member variable of the DartVM instance. Instead, all snapshots must be specified in the run configuration. For now, the new `Shell::Create` overload will only be used by Fuchsia till I refactor `dart_vm.cc`.
There are no API updates to the mobile shells.